Geographic variation in plant community structure of salt marshes: species, functional and phylogenetic perspectives.

In general, community similarity is thought to decay with distance; however, this view may be complicated by the relative roles of different ecological processes at different geographical scales, and by the compositional perspective (e.g. species, functional group and phylogenetic lineage) used. Coastal salt marshes are widely distributed worldwide, but no studies have explicitly examined variation in salt marsh plant community composition across geographical scales, and from species, functional and phylogenetic perspectives. Based on studies in other ecosystems, we hypothesized that, in coastal salt marshes, community turnover would be more rapid at local versus larger geographical scales; and that community turnover patterns would diverge among compositional perspectives, with a greater distance decay at the species level than at the functional or phylogenetic levels. We tested these hypotheses in salt marshes of two regions: The southern Atlantic and Gulf Coasts of the United States. We examined the characteristics of plant community composition at each salt marsh site, how community similarity decayed with distance within individual salt marshes versus among sites in each region, and how community similarity differed among regions, using species, functional and phylogenetic perspectives. We found that results from the three compositional perspectives generally showed similar patterns: there was strong variation in community composition within individual salt marsh sites across elevation; in contrast, community similarity decayed with distance four to five orders of magnitude more slowly across sites within each region. Overall, community dissimilarity of salt marshes was lowest on the southern Atlantic Coast, intermediate on the Gulf Coast, and highest between the two regions. Our results indicated that local gradients are relatively more important than regional processes in structuring coastal salt marsh communities. Our results also suggested that in ecosystems with low species diversity, functional and phylogenetic approaches may not provide additional insight over a species-based approach

Appendix B. Photographs of selected plant and herbivore species studied.

Photographs of selected plant and herbivore species studied

Salt marsh community dissimilarity (Bray-Curtis dissimilarity) of all pairs of sites in GA (Panel A), all pairs of sites in TX (Panel B), and all between-region pairs of sites (GA versus TX; Panel C).

<p>ANOVA results are shown for each classification approach. Salt marsh community dissimilarity was lowest in GA, intermediate in TX, and highest for the pairs of sites between GA and TX for all the classification approaches (Tukey HSD tests, <i>P</i> < 0.05).</p

Relationship between plot distance and plot dissimilarity (Bray-Curtis dissimilarity) for each of the 59 transects in GA and the 49 transects in TX, based on species (Panel A and D), functional groups (Panel B and E), and taxonomic groups (Panel C and F) respectively.

<p>In each panel, the lines represent linear regression fit to the data for each transect (detailed data for each transect are not shown). In each panel, the frequency histrogram of Mantel r values (ranges from 0 to 1) from Mantel tests for each transect is shown (significant Mantel r values evaluated by randomization tests are to the right of the dashed line).</p

Community compositional variation in salt marshes presented by the four most dominant plant species, functional groups, and taxonomic groups respectively in GA and TX regions.

<p>Distance from shore was normalized to 22 bins for each transect, and mean plant cover was calculated within each normalized bin for each species, functional, or taxonomic group.</p